On telecommunication connections, the transmission path used for transmitting signals is known to cause interference to telecommunication. This occurs irrespective of the physical form of the transmission path, whether the transmission path is e.g. a radio link, an optical fibre or a copper cable. Particularly in radio communication, situations often arise when the quality of the transmission path varies from one connection to another and during a connection.
Fading on a radio path is a typical phenomenon causing changes in a transmission channel. Other simultaneous connections may also cause interference, which may change as a function of time and place.
In a typical radio communication environment, signals between a transmitter and a receiver propagate along several paths. This multipath propagation is mainly caused by the signal being reflected from surrounding surfaces. Signals that propagate along different paths arrive at the receiver at different times due to a different propagation delay. Different methods have been developed to compensate for the fading caused by said multipath propagation.
One of the most efficient ways to compensate for fading on the radio path is adjustment of the transmission power of the transmitter. If the characteristics of the radio path are known, the power of the transmitter can be adjusted so as to cancel the effect of fading. However, in practice, such a solution is not easy to implement since, firstly, the transmitter should be aware of the quality of the channel, and real-time transmission of this information to the transmitter is difficult. Secondly, the transmission power limits set for transmitters and the dynamic ranges of transmitters bring about restrictions. Furthermore, power adjustment may itself result in inefficient transmission by increasing the power high in fade gaps.
A solution to the problem is to use diversity in the transmitter. In time diversity, interleaving and encoding are used to achieve time-based diversity in the signal to be transmitted. However, a drawback here is delays in transmission, especially when the channel is slowly fading. In frequency diversity, in turn, the signal is transmitted simultaneously at several frequencies. However, this is an inefficient method when the channel has a wide coherence bandwidth.
In transmission antenna diversity, the same signal or the different parts of the same signal are transmitted to a receiver over two or more antennas. Hereby the signal components that have multipath propagated through different channels are probably not interfered with by a simultaneous fade.
Publication WO 99/14871 discloses a diversity method wherein symbols, composed of bits, to be transmitted are encoded in blocks of a given length, and each block is encoded into a given number of channel symbols to be transmitted over two antennas. A different signal is transmitted over each antenna. For example, when bits to be encoded are split into two-bit blocks, the channel symbols to be transmitted are so formed that the channel symbols to be transmitted over the first antenna are composed of a first symbol and the complex conjugate of a second symbol, and the channel symbols to be transmitted over the second antenna are composed of the second symbol and the complex conjugate of the first symbol. However, the solution is presented only for use with two antennas.
Tarokh, V., Jafarkhani, H., Calderbank, A. R.: Space-Time Block Coding for Wireless Communication: Performance Results, IEEE Journal on Selected Areas in Communication, Vol. 17 pp. 451 to 460, March 1999, in turn, discloses similar solutions applicable to more than two antennas. The publication discloses an encoding method, which achieves full diversity at a ¾ code rate with four antennas.
Four desirable characteristics can be presented for what is known as open-loop-diversity:
1. Full diversity in relation to the number of antennas.
2. Only linear processing required in transmitter and receiver.
3. Transmission power is evenly divided among antennas.
4. As high code rate efficiency as possible.
The drawback in the solution presented above is that only 1 and 2 of the above-indicated prerequisites are realized. For example, the transmission powers of the different antennas are unevenly distributed, i.e. the different antennas transmit at different powers. This causes problems for the design of terminal amplifiers. Furthermore, the code rate is not optimal.